1. Field of the Invention
The present invention relates to a configuration of an illumination optical system of an endoscope or particularly to an illumination optical system that solves uneven illumination light.
2. Description of the Related Art
FIG. 6A shows an illumination optical system of a prior-art endoscope distal end portion described in Japanese Patent Laid-Open No. 5-119272 (Patent Document 1) and Japanese Patent Laid-Open No. 5-157967 (Patent Document 2), and in a distal end portion of an endoscope, an illumination lens including at least one positive lens 2, for example, is arranged on the distal (emitting end) side of a light guide (optical fiber bundle) 1 that leads light from a light source to the distal end portion of the endoscope. This positive lens 2 has at least one face as an aspherical face (h=fθ or h=f sin θ, where h: incident height to lens, θ: emission angle from lens, f: focal distance) and has an advantage that a light-amount loss is small and favorable illuminance distribution can be obtained in a wide angle.
FIG. 6B shows a configuration of the endoscope light-source portion, and light of a light source 3 in the figure has its light amount adjusted by an aperture diaphragm (leaf blade) 4 and is supplied to an incident end of the light guide 1 through a collective lens 5, and the light having passed through the light guide 1 is irradiated to an observed body through the positive lens 2 as the illumination light.
However, in the above-described illumination optical system having the positive lens 2, a phenomenon of unevenness (net-like unevenness) might be caused in the illumination light by uneven density in the optical fiber bundle constituting the light guide 1, and this illumination-light unevenness is particularly remarkable if an angular component of the light incident to the light guide 1 is biased, which is a problem.
That is, as shown in FIG. 6A, since a point P1 and a point P2 are in a conjugation relationship and an emission end of the light guide 1 is located in the vicinity of a conjugated position with an image forming point of the positive lens 2, for example, uneven density of the optical fiber bundle emerges as the illumination light unevenness. Moreover, in a light-source device of an endoscope, as shown in FIG. 6B, a light flux from the light source 3 is taken from one aperture diaphragm (leaf blade) 4, for example, and if a light component La of a light-flux outer periphery is large, light with a biased angular component is supplied to an incident end of the light guide 1, and the above illumination light unevenness is highlighted.
FIG. 7A shows light with a biased angular component and FIG. 7B shows light with a non-biased angular component, and according to the light in FIG. 7B, the illumination light unevenness is not remarkable but if the light with a biased angular component in FIG. 7A is increased, the illumination light unevenness becomes remarkable.
FIGS. 8A and 8B show a configuration of a prior-art illumination optical system with a purpose of solving the above-described illumination-light unevenness, in which FIG. 8A is shown in Japanese Patent Laid-Open No. 6-148519 (Patent Document 3), and Patent Document 3 discloses that a single fiber 7 is inserted between the emission end of the light guide 1 of the endoscope distal-end portion and the positive lens 2 so that the net-like unevenness of the light-guide fiber bundle is hardly shown. FIG. 8B is the one disclosed in Japanese Patent Laid-Open No. 2006-72098 (Patent Document 4), and Patent Document 4 discloses that a sand-blast 8A is worked on a projecting face of the positive lens 8, and a diffusion effect of this sand-blast face reduces uneven light distribution.
However, the configuration in FIG. 8A has nonconformity that the relatively long single fiber 7 is increased in the endoscope distal-end portion, which prolongs the distal end portion and the like, while in the configuration in FIG. 8B, use efficiency of the light supplied to the distal end portion through the light guide 1 is lowered, and even if an aspherical lens with high efficiency is designed for the positive lens 2 or the like, its advantage might be lost. Also, the lowered light use efficiency causes heat generation in the illumination optical system, which leads to the increase of the heat generation at the distal end portion, which is nonconformity. That is, in the endoscope, reduction in a diameter of the distal end portion and increase in pixels of an image pickup element are promoted in view of pain alleviation for a patient and improvement of observation performances, which tends to increase the heat generation in the distal end portion due to the decrease of radiation paths, increase in power consumption of the image pickup element and the like, and from this point of view, too, suppression of the heat generation in the illumination optical system is requested.
On the other hand, as a prior art in which light with a biased angular component is not to be supplied to the light guide, there is one as shown in Japanese Patent Laid-Open No. 7-49459 (Patent Document 5) and Japanese Patent Laid-Open No. 61-177416 (Patent Document 6) in which a light-source optical system is optimized by an exclusive mechanism and an angular component of light incident to the light-guide fiber bundle is controlled. However, such prior arts have a problem that the mechanism for optimization is complicated and expensive.
The present invention was made in view of the above problems and has an object to provide an illumination optical system of an endoscope that can favorably solve unevenness of illumination light with a simple and easy configuration without a need to change the optical system of the endoscope distal-end portion by eliminating incidence of light with a biased angular component to a light guide.